Description

The bug

World generation creates an isolated water block that floats in mid-air. From the height it looks like it's not a full source block, but when updating the area (e.g. by placing a torch), it spreads water from it.

It seems to be a permanent problem since I already had it when I generated a new world with that seed in previous pre-releases (and when converting a 1.12.2 world without that chunk with the same seed).